What size bike do I need?

Road, mountain, hybrid and kids' bike sizes by height, straight from Trek, Giant and Specialized's Australian size charts, plus how to measure yourself and what to do between sizes.

By the Ashburton Cycles workshop · Updated 1 October 2026 · 6 min read

The short version

Bike sizes are not standard. One brand's M is another's S, and road bikes are often sized in centimetres. Always check the size chart for the exact model.
At 175 cm you would ride a Trek Domane ML, a Giant Contend AR S or M, or a Specialized Tarmac 54 or 56: that is how much brands differ.
If you fit two sizes, a longer torso usually suits the larger one and a shorter torso the smaller one.
Size children's bikes by height and inside leg, not age, and do not buy big for them to grow into.
A test ride settles most sizing questions in minutes.

Why sizes differ between brands

Each brand draws its own frames, so the same rider can fit a different size on every bike. Trek now sizes road bikes with letters and shows the old centimetre size in brackets, Giant uses letters, and Specialized uses centimetres for road bikes, letters for cross-country and hybrid bikes, and S1 to S6 for trail bikes. Specialized says a rider around 173 cm could suit an S2, S3 or S4 trail bike, depending on how they ride.

The tables below use one popular model from each brand. Treat them as a starting point, then check the chart for the bike you are looking at. Giant labels every chart as a reference only and suggests confirming with a dealer.

Road bike sizes by height

Sizes whose published height range includes each height. Trek Domane SLR 7 Gen 5, Giant Contend AR 3, Specialized Tarmac.
Your height	Trek Domane	Giant Contend AR	Specialized Tarmac
155 cm	XS (47)	Below the XS range (starts at 157 cm)	44 or 49
160 cm	S (50)	XS	49
165 cm	M (52 to 54)	XS or S	52
170 cm	M (52 to 54)	S	52 or 54
175 cm	ML (56)	S or M	54 or 56
180 cm	L (58)	M or ML	56 or 58
185 cm	L (58)	ML or L	58
190 cm	XL (60 to 62)	L or XL	61

Mountain bike sizes by height

Trek Marlin 7 Gen 3, Giant Talon 1 (2025), Specialized Rockhopper.
Your height	Trek Marlin	Giant Talon	Specialized Rockhopper
155 cm	XS or S	XS	XS or S
160 cm	S	XS	S
165 cm	S or M	XS or S	S or M
170 cm	M	S or M	M
175 cm	M or ML	M	M or L
180 cm	ML or L	L	L
185 cm	L	L or XL	L or XL
190 cm	XL	XL	XL

For mountain bikes, Trek says reach (the horizontal distance from the bottom bracket to the head tube) is the most useful number for comparing sizes between brands. For cross-country bikes ridden seated, the effective top tube length matters more.

Hybrid and commuter bike sizes by height

Trek FX 3, Giant Cross City Disc 2 (2025), Specialized Crosstrail.
Your height	Trek FX	Giant Cross City	Specialized Crosstrail
155 cm	XS or S	Below the S range (starts at 160 cm)	S
160 cm	S	S	S
165 cm	S or M	S	S or M
170 cm	M	S or M	M
175 cm	M or L	M	M or L
180 cm	L	M or L	L
185 cm	L	L	XL
190 cm	XL	L or XL	XL

How to measure yourself

Height: barefoot, back against a wall.
Inside leg (inseam): in cycling shorts or thin clothes, stand barefoot with your back to a wall, pull a book or ruler up between your legs like a saddle, and measure from its top edge to the floor. Trek and Giant both describe a version of this.
Standover: with your feet about 20 to 25 cm apart, you want 2.5 to 5 cm of clearance between you and the top tube, as Cannondale suggests.

On a brand's geometry chart, reach is the horizontal distance from the pedals' axle (the bottom bracket) to the top of the head tube, where the handlebars attach, and stack is the vertical distance between the same two points. Two bikes with similar reach and stack fit in a similar way, whatever size each brand calls them.

If you are between sizes

A longer torso and arms usually suit the larger size, and a shorter torso the smaller one. Cannondale gives the same advice.
On a smaller frame you can add reach with a longer stem, but there is only so much a stem can do on a frame that is too big.
For trail and enduro bikes, riding style matters: a smaller size is easier to throw around, a larger one is steadier at speed.
Ride both if you can. A few minutes on each usually makes the choice obvious.

Kids' bike sizes

Kids' bikes are sized by wheel size. Giant advises against choosing by age, and Trek says not to buy a bigger bike for a child to grow into: a bike that is too big is harder to control and less fun to ride.

Trek's Australian kids' chart, converted from feet and inches. Brackets overlap, so check your child's inside leg too.
Wheel size	Age	Child's height
12 inch	2 to 3 years	91 to 99 cm
16 inch	4 to 6 years	104 to 122 cm
20 inch	5 to 8 years	114 to 137 cm
24 inch	8 to 11 years	124 to 150 cm
26 inch	10 to 12 years	142 cm and up

Giant's chart adds inside leg: a 20 inch bike suits an inside leg of about 50 to 61 cm, a 24 inch bike 59 to 74 cm, and a 26 inch bike 65 to 83 cm. Beginners should be able to put both feet flat on the ground from the saddle; more confident riders can manage on their toes. Trek suggests children over about 142 cm or 12 years can try adult frames.

Common questions

What size bike do I need for my height?

Use the tables above as a starting point, then check the chart for the exact model. Brands size differently: at 175 cm, for example, you would ride a Trek Domane ML, a Giant Contend AR S or M, or a Specialized Tarmac 54 or 56.

Is it better to size up or down?

If you have a long torso and arms, the larger size usually fits better; with a shorter torso, the smaller one. For mountain bikes, compare the reach figures on each brand's geometry chart.

What size bike does my child need?

Go by height and inside leg rather than age. As a guide, a 16 inch bike suits children about 104 to 122 cm tall, a 20 inch bike 114 to 137 cm and a 24 inch bike 124 to 150 cm. They should be able to reach the ground from the saddle.

Not sure between two sizes?

Come in to Ashburton Cycles and try both. It is the quickest way to know.

Visit Ashburton Cycles

Sources

Written by the workshop at Ashburton Cycles, 277 High Street, Ashburton VIC. We check every fact against the sources above and update the guide when rules or prices change. Last checked 1 October 2026.